THE TRANSPORTATION OF LIQUIDS
which the air may be removed by a vacuum pump. In long suction pipes where the air has more time to become liberated from the water the chamber may be constructed as a separating box.
The object of a foot valve is to keep the pump primed during a stoppage. It adds to the friction and reduces the possible suction lift. Figure 29 shows a clack valve and strainer. Figure 30 shows one fitted with a number of °™Ql1 rh'°" valves. If the water carries impurities tha'u . damage to the pump a strainer should be provide, suction pipe.
FIG. 28.—Vacuum chamber.
FIG. 29.—Clack valve and strainer.
The ideal suction lift of a pump equals the water column sustained by the atmospheric pressure minus the vapor pressure; the former is affected by the barometer and altitude, the latter by the temperature of the water. The hydraulic losses, which comprise the friction through the suction pipe, valves and
FIG. 30,—Foot valves.
passages, and the velocity head are generally insignificant as compared with the influence of the air contained in the suction water, the effect of which can not be computed. Figure 31 is an empirical diagram, giving the ideal suction lift t, the attainable suction lift a for sea level, the suction head required to make the water flow into the pump for high temperatures and the maximum possible suction lift under favorable conditions m, i.e., when the suction pipe is tight and the water does not carry an excessive amount of air. Curves a and m are based on a velocity of 3 ft. per second and a short suction pipe with one bend. The suction lift is measured vertically from the water in the well to the underside of the